I agree on removing everything. 4 things that cause bindings, you have to check them all individually, 1:lever action on heater control, 2: switches/vac modules, 3:cables, 4: heater box arms.
Once you find which one(s) are binding, fix those. If cables are rusty internally, maybe some evaporust? then a thin lube like slick 50 to free them up. my .02